Systems, more particularly used in automotive electronics, which systems are networked by serial bus systems, are generally sensitive to ground offsets between the component parts; this especially holds for bus systems supporting one-wire operation such as, for example, error-tolerant CAN (Controller Area Network) physical layers. If in such systems the ground offset between the component parts becomes too large, there will be interference or even a breakdown of the communication.
According to the state of the art the bus lines themselves are used at present to detect from their idle level whether there is a deviation from the nominal level or not. The idle state of the bus lines results from the superpositioning of all the components in the network system and allows a comparison with the local supply voltage. When a predefined limit value is exceeded, it should be assumed that the local ground connection is no longer optimal and a warning may be issued.
For the correct evaluation of the bus signal it is now important for the bus to be in its idle state and just then no active transmissions take place over the bus; when a user is actively sending, a comparison with the local supply will always lead to a wrong result. To avoid this, document WO 97/36184 A1 proposes to utilize the digital transmission signal (TX) to ensure that the bus is in its idle state at the time of measurement.
However, this method has the disadvantage that according to protocol it cannot be excluded that the bus is not in its idle state at the time of measurement. Especially with the Controller Area Network (CAN) protocol the bus may also be in its active state (acknowledge bit/arbitration phase/error flags) also without a transmission signal (TX) being present, and falsify the measurement of the ground offset in this way; this leads to an erroneous indication of ground defects.